Go uses a user space scheduling model whereby a large number of goroutines are mapped onto a small (usually on the order of the number of cores your operating system exposes) number of operating system threads. When a goroutine executes time.Sleep, it is no longer runnable (obviously) and as part of going to sleep to yields back to the scheduler so that the thread can service other runnable goroutines.

It is possible (although I can't test it as I don't have access to a windows system) that over time calling windows.GetCurrentThreadId() will return essentially random values. To avoid this some users call runtime.LockOSThread to reserve the current thread for only the runnable goroutine.

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Thanks for your answer, It is totally obvious for me. But I would like to ask a question another. I know here is not place for discussion. Let's say we have a running go routine in a thread. Does Go allow to be executed another go routine in the same thread before running thread does not end? Do I have to use runtime.LockOSThread function to reserve the thread?